“Nanotechnology” refers to technologies that observe, fabricate, and utilize minute structures of around one hundred-millionth of a meter (10−8m=10 nm) in size.
High-precision microscopes called “scanning tunnel microscopes” were invented in the latter half of the 1980s, thereby making it possible to see single atoms and single molecules. By using a “scanning tunnel microscope”, it is possible not only to observe atoms and molecules, but also to manipulate individual atoms and molecules.
For example, there have been reported examples where atoms are arranged on the surface of a crystal to write characters. However, although it is possible to manipulate atoms and molecules, it is not realistic to individually manipulate a huge number of atoms and/or molecules to assemble a new material or device.
To form a structure of nanometer size by manipulating atoms, molecules, or groups thereof, new ultraprecise machining technology capable of such manipulation is required. As ultraprecise machining technology with nanometer precision, methods that can be roughly classified into two types are known.
A first of these technologies has conventionally been used in the manufacturing of various semiconductor devices, and is a so-called “top-down” method where for example a large silicon wafer is precisely carved to the smallest possible extent to produce integrated circuits in the silicon. The other technology is a so-called “bottom up” method where microscopic units such as atoms and molecules are the components and the desired nanostructures are fabricated by assembling such small components.
For the top-down method, the well-known “Moore's Law” announced in 1965 by the cofounder of Intel Corp. Gordon Moore exists regarding the limit on how small structures can be fabricated. Moore's Law states that transistor density doubles every eighteen months. Since 1965, the semiconductor industry has raised the transistor density in keeping with Moore's Law for more than thirty years.
The International Technology Roadmap for Semiconductors (ITRS) for the semiconductor industry in the coming fifteen year period that has been announced by the Semiconductor Industry Association (SIA) expresses the opinion that Moore's Law will continue to be valid.
The ITRS is composed of a short-term roadmap to the year 2005 and a long-term roadmap to the year 2014. In the short-term roadmap, the process rule of semiconductor chips will reach 100 nm and the gate length of microprocessors will reach 65 nm in 2005. In the long-term roadmap, the gate length in 2014 will reach 20 to 22 nm.
As semiconductor chips become miniaturized, the processing speed increases and at the same time power consumption is suppressed. In addition, the number of products that can be produced from a single wafer increases, so that the manufacturing cost also falls. For these reasons, microprocessor manufacturers are competing to improve the process rule and transistor density of new products.
In November 1999, a research group in the USA announced breakthrough results for miniaturization technology. This breakthrough was a method of designing gates on a FET (Field Effect Transistor) called “FinFET” that was developed by a group led by Professor Chenming Hu, Head of Computer Science at the University of California, Berkeley. This method makes it possible to form four hundred times as many transistors on a semiconductor chip compared to conventional methods.
A gate is an electrode that controls the flow of electrons in a FET channel, with current designs typically having a structure where the gate is disposed in parallel with the surface of a semiconductor and controls the channel from one side thereof. With this structure, it is not possible to shut off the flow of electrons unless the gate has at least a predetermined length, and therefore gate length has been thought of as one factor limiting the miniaturization of transistors.
On the other hand, with FinFET technology, a gate has a fork shape so as to straddle both sides of the channel and so can effectively control the channel. With a FinFET structure, the gate length and transistor can be made even smaller than conventional structures.
The gate length of a plot-type FET manufactured by the above research group is 18 nm, which is around one tenth of the current typical gate length and matches the year 2014 size given in the long-term roadmap of the ITRS. The research also states that half of this gate length is possible. Professor Hu's group has not applied for patents in expectation that the technology will be widely adopted by the semiconductor industry, and so FinFET may become the principal manufacturing technology.
However, “Moore's Law” also states that a limit will eventually be reached due to natural laws.
For example, according to current mainstream semiconductor technology, a circuit pattern is burnt into a silicon wafer by lithography to manufacture semiconductor chips. To achieve further miniaturization, the resolution needs to be raised, but to raise the resolution, it is necessary to realize technology that uses light of a shorter wavelength. Since there is a physical limit on the wavelength of the light used in lithography, a breakthrough from another angle is required to exceed this limit.
Also, when the packing density is increased, there is also the risk of the amount of heat emitted per semiconductor chip becoming too large, causing the semiconductor chips that have reached a high temperature to malfunction or to become thermally damage.
In addition, according to predictions made by specialists, it is believed that if the semiconductor industry continues to reduce the size of chips, there will be a large increase in equipment costs and process costs, and yields will also worsen, making the manufacturing of chips economically unfeasible in around 2015.
In this way, as a new technology that can break through the technical wall facing the top-down method described above, attention has been focused on research that provides individual molecules with functions as electronic components. Electronic devices (molecular switches and the like) composed of single molecules are fabricated using the bottom-up method.
Research that produces nanometer-sized structures using the bottom-up method is being carried out for metals, ceramics, and semiconductors. However, there are several million types of molecules that have different shapes and different functions and are fundamentally independent from one another, and by utilizing such molecules, using the bottom-up method it is possible to design and fabricate devices (molecular devices) with completely different characteristics to conventional devices.
For example, the width of conductive molecules is just 0.5 nm. A wire material of such molecules can realize wires of several thousand times the density compared to the width of wires (around 100 nm) realized by current integrated circuit technology. Also, as one example, if a single molecule is used as a memory element, it is possible to record 10,000 times or more than a DVD.
Unlike conventional semiconductor silicon, molecular devices are synthesized by a chemical process. In 1986, Hiroshi Koezuka of Mitsubishi Electric discovered the world's first organic transistor made of polyolefin (a polymer).
In addition, a research group composed of Hewlett Packard (HP) Corp. and the University of California at Los Angeles (UCLA) succeeded in manufacturing an organic electronic device and in addition to announcing their results in “Science” Magazine (July 1999 edition) applied for patents (See U.S. Pat. No. 6,256,767B1 and U.S. Pat. No. 6,128,214). This research group produced switches using a molecule film composed of several million organic rotaxane molecules, and by connecting these molecular switches, fabricated AND gates that are fundamental logic circuits.
A collaborative research group of Rice University and Yale University succeeded in producing a molecular switch that carries out a switching operation where the molecular structure is changed by introducing electrons when an electric field is applied, and announced their results in the November 1999 edition of “Science” Magazine (see J. Chen, M. A. Reed, A. M. Rawlett and J. M. Tour, “Large On-Off Ratios and Negative Differential Resistance in a Molecular Electronic Device”, Science, 1999, Vol. 286, 1552-1551 and J. Chen, M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin and J. M. Tour, “Conductance of a Molecular Junction”, Science, 1997, Vol. 278, 252-254). A function whereby the device can be repeatedly turned on and off was not realized by the HP/UCLA group. This is a basis for producing a small, high performance computer with one millionth of the size of a normal transistor.
Professor J. Tour (Rice University, Chemistry Department) succeeded in synthesis and has made it possible to reduce the manufacturing cost of a molecular switch to one several thousandth of the conventional cost by making the costly clean room used in conventional semiconductor manufacturing unnecessary. It is expected that hybrid computers composed of molecules and silicon will be manufactured within five to ten years.
In 1999, Bell Labs (Lucent Technology) fabricated an organic thin-film transistor with a pentacene single crystal that exhibits characteristics that match those of an inorganic semiconductor.
Even though much research is being conducted into molecular devices that function as electronic components, the research conducted into molecular devices so far has mostly been into devices driven by light, heat, protons, ions, or the like (see, for example, “Molecular Switches”, edited by Ben L. Feringa, WILEY-VCH, Weinheim, 2001), and research into devices driven by electric fields has been limited.
As conventional molecular elements driven by an electric field, there are only elements that use changes in the solid-state properties of the molecules themselves upon which the electric field has acted, that is, elements where the molecules themselves are thought of as single elements and the electron state of the molecules is changed by the electric field. For example, with an organic FET, by changing the electric field that acts upon organic molecules in the channel region, carrier mobility in the organic molecules is modulated.
In view of the situation described above, it is an object of the present invention to provide a functional molecular element and a functional molecular device whose functions can be effectively controlled by an electric field based on a new principle.